1. Technical Field of the Invention
The present invention relates generally to communications systems and more particularly to methods and apparatus for utilizing the bandwidth of a communication channel more effectively.
2. Description of Related Art
Communication systems generally have at least two entities communicating with each other through a medium (channel). In some systems, one entity may be a transmitter while the other entity is a receiver, enabling one-way communication. In other systems, two-way communication may be enabled by the entities being capable of both transmitting and receiving, one transmitter sending communication signals to at least one receiver through some kind of medium or channel. In the context of a computer-based communication system, two or more computers may communicate with each other where a transmitting computer sends data to a receiving computer. While many possible channels exist for communicating between computers, they generally fall into two categories, wireline and wireless.
Multi-Channel Modulation
Multi-channel modulation (MCM) is a generic term describing a technique for dividing the entire bandwidth of a channel into multiple subcarriers, where each subcarrier does not interfere with the others. When the subcarriers are used to represent different frequencies (referred to as “tones”), this technique is referred to as frequency division modulation (FDM), and if each tone is orthogonal to the others (so that intertone interference is minimized), then this technique is referred to as orthogonal frequency domain modulation (OFDM). One example of an OFDM system is the IEEE 802.11 g wireless local area network (WLAN) standard.
The spacing between adjacent tones preferably is selected to be the inverse of the symbol period so that each tone is orthogonal and non-interfering with other tones. Note while the tone spacing may be larger, the inverse of the symbol period represents a lower limit of tone separation in that spacing the tones closer than this value may induce intertone interference. Information is transmitted by modulating the phase and amplitude of each tone. Typically each tone is modulated with a symbol from a discrete set such as a phase shift keying (PSK) or quadrature amplitude modulation (QAM) constellation.
One popular implementation of MCM used in telecommunications systems is in digital subscriber lines (DSLs). These services provide data bandwidth to subscribers for services such as broadcast video, video on demand, as well as many other bandwidth intensive services. The physical implementation of DSL involves a wireline connection between the telecommunications provider and the subscriber. These wireline channels often include copper loops used in telephone subscriber lines. Due to varying channel conditions, information capacity of each tone may vary. For example, the path loss from the transmitter to the receiver for each tone may not be constant. In part, this is due to the receiver receiving both the transmitted signal and the reflections of the transmitted signal, which causes deleterious reception of the transmitted signal.
Channel Optimization
Because the tones within the channel may have different gain and distortion characteristics, the information capacity of each tone tends to vary. The aggregate capacity of the channel may be increased by allocating different power and/or bits to each tone. The maximum data capacity of a tone is a function of the channel gain and the power used in transmit on that tone. As such, the optimal power distribution for the channel must be achieved to reach the channel capacity. Power may be allocated uniformly as an engineering tradeoff, but unless the gains on each tone are equal, this will result in some loss. Bits must also be allocated optimally on each tone. One method for efficiently allocating the data to each tone of the channel is to use a “bit loading” algorithm, which allocates an appropriate number of bits to each tone based on that tone's characteristics (such as distortion), in order operate as close to the channel capacity as possible, and thereby optimize the use of the resource.
In DSL applications, MCM techniques may be used to optimally adapt the communications channel so that each tone may carry a different QAM size constellation (i.e., bit loading) based on the channel's measured signal-to-noise ratio (SNR). In performing the bit loading, the SNR of each tone is estimated during a training phase when the system is initially configured. Based on this estimation, the appropriate bit loading is assigned to the tones within the channel to maximize the channel's throughput. In a wired communications system such as DSL, the channel is considered stable because the channel's characteristics do not vary with time. Implementing MCM in stable or static channels allows the channel characteristics to be estimated when the system is initially configured so that bit loading may later be performed across the tones.
MCM techniques are also used in wireless systems, such as in wireless local area networks (WLANs) employing the 802.11 standard. However, wireless systems are not amicable to bit loading optimization techniques because their transmission characteristics may vary with time and are thus considered unstable. For example, a laptop computer communicating with a stationary computer using a wireless connection may be moved to different geographic locations within the WLAN at different times. In such systems, initial estimates of a tone's characteristics (e.g., SNR) are subject to change so that wireless standards, such as the 802.11 standard, do not support bit loading techniques. Techniques for employing channel optimization, such as bit loading, in wireless communications systems would be highly desirable because these techniques would enable the system to operate closer to optimum capacity. Yet, implementing channel optimization techniques may be difficult.